Spawning migration of Atlantic salmon (Salmo salar) in a regulated river
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Atlantic salmon (Salmo salar) are an iconic species that undertake arduous migrations between marine and freshwater habitats to complete their life-cycle. Populations of Atlantic salmon are in decline across their range and a number of factors have been implicated. Hydropower dams block migration routes and disrupt longitudinal connectivity in rivers. Diverse fish passage solutions have been implemented worldwide to improve river connectivity, but with varying degrees of success. This study reviews the historical literature and outlines the negative impacts of a hydropower scheme on the salmon stocks of a large river. It was evident that a number of anthropogenic activities, including habitat degradation and over-exploitation, significantly contributed to the decline of salmon stocks. The upstream migration behaviour and route selection of spawning Atlantic salmon on a large regulated river was evaluated. A radio telemetry study revealed that extended delays in migration were experienced and the majority of salmon were attracted to the hydropower station. The passage rate recorded at a Borland fish lock located at the hydropower dam was 6.1%. The passage rate recorded at a pool and weir fish pass located along a bypass route was 33.3%. A greater proportion (87.8%) of wild salmon were recorded migrating via the hydropower route. Upstream counts of salmon were related to acoustic camera observations at the downstream entrance of the fish lock. Both telemetric and acoustic camera data recorded high salmon activity at the Borland lock entrance suggesting that the fish pass is situated in a suitable location. Further investigation of the fish passage facilities may yield a better understanding of the processes involved and improve the low passage rates recorded in this study. The migration strategies of salmon populations on two regulated rivers were evaluated. Differences between the two river systems were apparent, emphasising the importance of incorporating river-specific knowledge into salmon management protocols.